The present invention relates to a method and apparatus for propelling and retarding diesel electric haulers. Such haulers include very large off-road dump trucks, typically having a load carrying capacity of 100 to 200 tons or more. They are used most often in open pit mining operations and other applications where very large amounts of earth, rock, etc., must be moved.
Many such haulers currently utilize a diesel/electric drive as their power source to propel the vehicle. Such systems typically are comprised of a prime mover (diesel) driven electric generator whose prime output is used for supplying electric power to high horse power DC electric motors connected through a gear reduction to a set of drive wheels on each side of the vehicle. Typically, the electric power is generated as alternating current ("AC") and converted into direct current ("DC") for use by DC wheel motors. Many control schemes have been designed to regulate the power and speed of these motors to provide adequate power output to propel the haulers. Since DC wheel motors require variable voltage from zero to a maximum as vehicle speed increases, and a variable current to control torque output, all control schemes must include means to regulate these parameters.
Propulsion power must be supplied when the haulers operate on level or upwardly sloping surfaces. When operated on downwardly sloping surfaces, no propulsion power is needed; instead, means must be provided for retarding the forward progress of the haulers. Friction brakes are not suitable for this purpose, since they tend to wear out quickly due to the very large mass of these vehicles, especially when loaded. While friction or similar braking systems are included on the haulers as the primary stopping means, most such haulers utilize the DC electric wheel motors to provide the continuous retarding torque required when traveling on a downward slope. This is accomplished either by reversing the wheel motor field current or armature current, causing the DC motors to reverse torque direction and act as DC generators, powered through the gear reduction units by the hauler drive wheels. Resistance sources, called "retard resistors," are used to create a load, so that current generated by the DC motors is consumed by the resistors and dissipated as heat into the atmosphere. The amount of current consumed creates a corresponding load on the DC wheel motors, which is transmitted through the gear reduction units to the drive wheels as retarding torque.
Even when operated in the retarding mode, it is necessary on prior art haulers for the prime mover diesel engines to continue to generate sufficient power to meet auxiliary and parasitic power requirements on the hauler for such items as driving fans, powering hydraulic systems, generating field current for the wheel motors, powering the air conditioning or heating, etc. Also, even an unloaded engine uses fuel to overcome losses due to compression, friction, and the like. Such auxiliary and parasitic loads and engine losses result in substantial consumption of fuel by the engine even when the hauler is operating in a retarding mode in which it is generating, and dissipating to the atmosphere, substantial quantities of power.
In some prior art haulers, such as the Marathon LeTourneau Model T-2000 TITAN truck, means are provided for regenerating at least a portion of the DC power generated by the DC electric wheel motors when operating in a retarding mode into the AC power system, so as to relieve all or part of the AC power load on the AC generator and to drive the AC generator as an AC synchronous motor to provide power to the engine to relieve all or part of the mechanical engine loads. However, such systems heretofore have not been capable of supplying the first power generated by the DC electric drive motors into the AC system or consistently to supply power into the AC system throughout the retard cycle because of the operational characteristics of the retard resistors and motor voltage and current limits. Therefore, even with the best existing designs, it often is necessary to consume fuel to power engine parasitic and auxiliary loads even while the overall system is dissipating large amounts of power as heat from the retarding resistors.
Because of the high cost of fossil fuels, and because of engine horse power limitations, it also sometimes is desirable to operate off-road haulers and similar vehicles on trolley systems whereby an external source of electric power, such as a trolley line, is used to power the electric DC wheel motors. Since the trolly line power source is a fixed voltage source, control means must be implemented to regulate the voltage and current required by the DC electric wheel motors. Trolley power often is available for only a portion of the area in which the haulers operate (for example, trolley power may be available on roads into or out off the mining area, but not within the floor or other parts of the open pit mine itself). Therefore, it is necessary to continue to provide on-board diesel electric power for the haulers. The haulers thus may be propelled by on-board generated power in areas where trolley line power is not available, and switch to trolley line power for other portions of their load cycle. This requires means for accommodating both on-board generated electric power and fixed voltage external DC power from the line trolley. This type of operation typically is referred to as "trolley-assist" since the trolley line power is available only in certain locations to assist the hauler operation.
On existing haulers using trolley-assist, mechanical switching systems are provided so that the hauler's DC wheel motors are decoupled from the vehicle's on-board diesel electric system and are coupled to, and controlled by, an alternate power control means which regulates the voltage and current from the trolley line power to the DC wheel motors. Where trolley line power is not available, the DC wheel motors are recoupled to the on-board diesel electric system. This creates an "either/or" operating choice and imposes stringent limitations on operating procedures. The mechanical switching gear and the alternate voltage and current controls required for trolley operation also add to capital costs and maintenance costs. Moreover, with existing trolley-assist systems, even when the hauler is operating on trolley line power, the on-board engine must be kept running to provide power for auxiliary functions of the vehicle.
It is, accordingly, a primary object of the present invention to provide a propelling and retarding system for off-road haulers in which the initial power generated in the retarding mode can be regenerated into the AC electric system, to replace loads otherwise carried by the internal combustion engine.
Another object is to provide such a system in which essentially all power requirements for the internal combustion engine and AC power system are met before any power generated in the retard mode is dissipated through the retarding resistors.
A further object is to provide such a system in which auxiliary line power at fixed voltage may be fed into the system for powering the DC electric wheel motors without first decoupling the motors from the on-board diesel electric system.
A further object is to provide such a system which can utilize auxiliary line power over a wide fixed voltage range, from at least approximately 1,000 to at least approximately 2,000 volts.
A still further object is to provide such a system in which trolley line power may be utilized to power the hauler's DC wheel motors in the propel mode, to meet all other electrical power requirements for the hauler and to supply power to the internal combustion engine to relieve all auxiliary and parasitic loads on the engine, so as to eliminate essentially all fuel requirements for the engine during the periods when trolley line power is available.